Catalyst activation processes generally comprise heating a catalyst in the presence of an activating gas. The catalyst is usually activated as a fluidised bed, the fluidisation being achieved by passing the activating gas through a grid, known as a fluidisation grid, to fluidise the bed of catalyst particles held above the fluidisation grid.
A grid plate and catalyst activation using such a fluidisation grid plate are described, for example, in U.S. Pat. No. 3,829,983.
More recently, a catalyst activator vessel has been described in WO 2004/024312. According to this document, problems in scaling up catalyst activator vessels can be solved by specific design of the activator vessel fluidisation grid, which thereby allows diameters of greater than 50″ (1.27 m) to be achieved. In particular, the activator vessel fluidisation grid plate can have conical depression which overlap by at least 17%, where the percentage overlap is defined as the amount, expressed as a percentage, by which the nominal diameter of a depression exceeds the centre-to-centre spacing between one depression and the nearest adjacent depression.
In general, the depressions in a fluidisation grid plate are formed in a triangular arrangement by drilling cones into a metal surface as described in WO 2004/024312. In such an arrangement each depression has 6 nearest neighbours spaced around it, and as neighbouring cones overlap they form an array of regular hexagons at the surface.
It can be shown that for an array of regular hexagons at a surface the percentage overlap in that surface is 15.5%. To achieve an overlap of greater than 17%, WO 2004/024312 defines a nominal upper surface in which the nominal diameter is measured.
In effect, it is necessary to lower the surface in which the hexagons sit relative to the original surface of the plate in which the depressions are drilled by drilling further down into the original plate. This is shown in FIG. 4 of WO 2004/024312 as the nominal surface 52 which corresponds to the original surface of the plate, and which is reproduced as FIG. 2 herein. The array of regular hexagons as viewed from above does not change, but the depth of the surface in which the hexagons reside lowers relative to the original surface.